Bumping gaia.json for 2 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/2b1233dfd4b6
Author: Jared Hirsch <ohai@6a68.net>
Desc: Merge pull request #19507 from 6a68/bug-993794-remove-fxa-pref-settings

Bug 993794 remove fxa pref settings

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https://hg.mozilla.org/integration/gaia-central/rev/75f5d346506f
Author: Jared Hirsch <ohai@6a68.net>
Desc: Bug 993794 - Remove Firefox Accounts pref from FTU and Settings. r=arthurcc,arcturus
Bumping manifests a=b2g-bump